At present, human beings are facing serious environmental destruction on a global scale. For example, sulfur oxide (SOx), nitrogen oxide (NOx), and particulates, which are generated by combustion of fossil fuel such as petroleum or coal, are released into the atmosphere to destruct a global environment remarkably. In particular, sulfur oxide causes acid rain, destructs an environment such as a forest or a lake, and makes a large influence on ecosystem.
The motor vehicle industry is extensively developing a technology for addressing exhaust gas, and is promoting research and development of novel technologies such as a combination of high-pressure injection and exhaust gas recirculation (EGR), homogeneous charge intelligent multiple injection, and an NOx catalyst. A concentration of sulfur in gas oil is decreased in order to alleviate influences on application of the EGR effective for a decrease in NOx and an aftertreatment apparatus for removing particulates.
This EGR is a technology for recirculating exhaust gas to decrease a concentration of oxygen in an engine and to make a combustion temperature lower, to thereby decrease production of NOx, and a cool EGR using an EGR cooler is more effective. Also in this technology, which has already been put into practical use in an gasoline engine, when a content of sulfur in a fuel is large, sulfur is accumulated as sulfuric acid in an engine and abrades engine parts such as a cylinder and a piston ring, and hence it is necessary to decrease a concentration of sulfur. Further, SO2, which is produced by combustion of sulfur, serves as a poisoning substance for an oxidation catalyst for treating a soluble organic fraction (SOF: solvent soluble content) in particulates or a reduction catalyst for NOx. From this viewpoint as well, there is a demand for a decrease in concentration of sulfur in gas oil as fuel.
Constituents of the particulates are a sulfate derived from a sulfur compound in gas oil, soot (carbon; black smoke), and an SOF, and contents of the soot and the SOF are substantially the same. Gas oil subjected to deep desulfurization (sulfur concentration: 500 ppm) has substantially no sulfate content and is constituted only of the soot and the SOF. However, SOx, which is produced by combustion of sulfur, serves as a poisoning substance for the oxidation catalyst for treating the SOF or the reduction catalyst for NOx. From this viewpoint as well, there is a demand for an additional decrease in concentration of sulfur.
From such viewpoint, there is a particularly strict regulation on a content of sulfur in a petroleum fraction such as gasoline, kerosene-gas oil, or fuel oil, and there is a demand for development of a hydrotreating catalyst having an excellent activity of efficiently removing sulfur in a petroleum fraction.
A hydrodesulfurization catalyst for removing sulfur in a petroleum fraction, which is used industrially at present, is generally obtained by supporting molybdenum or tungsten and cobalt or nickel on a porous alumina support. It is known that a desulfurization activity of such hydrodesulfurization catalyst is remarkably affected by a supported state of a catalytic metal on a support. As a method involving improving the supported state to improve the activity of the hydrodesulfurization catalyst, there is known a hydrodesulfurization catalyst using a porous titania support, having a relative desulfurization activity increased by 2-fold or more as compared to a catalyst using an alumina support.
However, titania has drawbacks such as having a small specific surface area, having poor formability, and having low mechanical strength as compared to alumina. In addition, titania is high in raw material price and thus is economically disadvantageous as compared to alumina. Hence, titania is seldom used industrially as the hydrodesulfurization catalyst.
Various studies have been made in order to overcome those drawbacks of the titania support. For example, Patent Literature 1 proposes a method involving increasing a specific surface area of titania and increasing a desulfurization activity. This method includes adding an anion or a cation as a particle growth inhibitor to a titanium hydrous oxide hydrosol or hydrogel manufactured by a pH swing method or a dried product thereof, and drying and calcining the mixture, thereby giving a high-performance hydrodesulfurization catalyst excellent in thermal stability, having a large specific surface area, containing a highly dispersed catalytic metal, having an improved catalytic activity, and having high mechanical strength. However, also in this method, such economic disadvantages that a titania raw material cost is high, and a mass of a catalyst to be filled per volume of a reactor becomes large because of a large compact bulk density of the catalyst have not been overcome yet.
In view of the foregoing, in order to provide a catalyst not only excellent in economic efficiency but also excellent in performance as a hydrodesulfurization catalyst (i.e., having high activity and excellent mechanical strength), various studies have been made on realizing the catalyst using a complex of alumina, which is low in raw material cost, and titania, which can be expected to exhibit high performance. For example, Patent Literature 2 discloses a technology according to a manufacturing method for a catalyst support for a hydrorefining treatment using a complex of an aluminum ion and a titanium ion formed by coprecipitation. Further, for example, Patent Literature 3 discloses a manufacturing method for an alumina/titania complex catalyst support involving adding a titanium hydroxycarboxylic acid salt and/or a titanium oxide or hydroxide sol and hydroxycarboxylic acid to aluminum oxide and/or hydroxide, and kneading and calcining the mixture.
However, in those technologies, improvements are found in terms of economic efficiency and catalytic strength by the addition of alumina, but in terms of catalytic activity, a content of titanium oxide decreases, and only performance depending on a mixing ratio between titanium oxide and alumina is exhibited.
As a method of overcoming such drawbacks, for example, Patent Literature 4 discloses a method involving introducing tetrachloride titanium gas into an alumina support to carry out chemical vapor deposition of titanium on a surface of alumina, thereby apparently coating a pore surface of the alumina support with titania. However, this method uses gaseous titanium tetrachloride. Hence, it is necessary to repeat the operation in order to increase an addition amount of titania, and there is a problem in terms of industrial productivity. In addition, owing to a reaction of TiCl4 and H2O, HCl gas is inevitably generated, and hence it is necessary to take countermeasures for corrosion of a manufacturing facility and environmental pollution as well into consideration.
Further, for example, Patent Literature 5 discloses a method involving impregnating an alumina support with a solution containing titanium to coat a pore surface of alumina with titanium. However, this method also has a problem in terms of industrial productivity because it is necessary to repeat an impregnation operation and a drying or calcination operation in order to increase an addition amount of titania. Further, the solution containing titanium is impregnated into pores of the alumina support, and hence physical properties such as a pore volume and a specific surface area of the alumina support inevitably deteriorate. Thus, it is difficult to improve performance of a hydrodesulfurization catalyst to a large extent.
Meanwhile, for example, Non Patent Literature 1 discloses a method involving providing a titania-alumina support by precipitation of titanium hydroxide on a surface of an alumina hydrate particle (coating), followed by aging, filtration, washing, forming, and calcination. However, the method according to the technology has a drawback in that an aggregate of titania is produced when an amount of titanium is more than 9.1% by weight, resulting in decreases in specific surface area and pore volume of alumina, although thermal stability and mechanical strength are improved.
The inventors of the present invention have already disclosed a catalyst manufacturing technology capable of manufacturing a catalyst which is excellent in specific surface area and mechanical strength and has a hydrodesulfurization catalyst activity comparable to that of a titania hydrodesulfurization catalyst based on such a phenomenon that an inorganic oxide and titanium oxide are chemically and microscopically integrated with each other even when titanium oxide is supported in an amount of 13% by mass or more by precipitating and stacking titanium oxide between both isoelectric points of the inorganic oxide and titanium oxide in supporting titanium oxide on a surface of the inorganic oxide (Patent Literature 6).
In addition, various studies have been made on an improvement in activity of a hydrotreating catalyst by using an organic compound in supporting a catalytic metal on a support.
For example, Patent Literature 7 discloses a manufacturing method for a hydrotreating catalyst involving impregnating an alumina support with a catalytic component-containing aqueous solution containing a catalytic metal, phosphoric acid, and additives including a dihydric or trihydric alcohol having 2 to 10 carbon atoms per molecule, an ether thereof, a monosaccharide, a disaccharide, and a polysaccharide, and drying the support at 200° C. or less.
Further, Patent Literature 8 discloses a manufacturing method for a hydrotreating catalyst having a catalytic metal supported on a support obtained by supporting an aqueous solution containing a titanium compound on an alumina hydrogel, followed by calcination. The “Detailed Deception of the Invention” section of the patent literature describes that a water-soluble organic compound is preferably added to a catalytic component-containing aqueous solution. In addition, the literature mentions, as the water-soluble organic compound, for example, a diol, an alcohol, an ether group-containing water-soluble polymer, a saccharide, and a polysaccharide each having a molecular weight of 100 or more and having a hydroxy group and/or an ether bond.
None of those two patent literatures discloses any organic compound particularly effective for a catalytic activity.